A robotics lab in Slovenia has been conducting experiments to assess human-robot pain thresholds by allowing robots to repeatedly strike human volunteers in the arm with blunt and sharp tools.

Borut Povše, a robotics researcher from the University of Ljubljana, has received "ethical approval" to allow this type of research. He has enlisted six colleagues to participate in the study, where each is to endure physical pain from robots.

"We are taking the first steps to defining the limits of the speed and acceleration of robots, and the ideal size and shape of the tools they use, so they can safely interact with humans," said Povše.

Povše borrowed a production-line robot from the Japanese technology firm Epson, and programmed its arm to travel toward a specified point that is occupied by the human's arm, so that the robot will have to hit it. Each volunteer was struck 18 times by a robot arm that contained two tools at the end of it - one that was round and blunt, and the other that was sharp.

Throughout this process, volunteers were asked to rate the pain that they've endured by each individual hit. Their answer choices were painless, mild, moderate, horrible or unbearable pain. Povše tested the robot on himself first, then allowed the robot to begin striking his colleagues. After all six participants received their 18 strikes, most of them rated the pain as either "mild" or "moderate."

"Determining the limits of pain during robot-human impacts this way will allow the design of robot motions that cannot exceed these limits," said Sami Haddadin from DLR, the German Aerospace Centre in Wessling.

While this type of research has many benefits associated with the improvement of robot-human interaction, other researchers such as Michael Liebschner, a biomechanics specialist at Baylor College of Medicine in Houston, Texas, believe that more work is needed to determine the effectiveness of these experiments.

"It makes sense to study this," said Liebschner. "However, I would question using pain as an outcome measure. Pain is very subjective. Nobody cares if you have a stinging pain when a robot hits you - what you want to prevent is injury, because that's when litigation starts."

The next step for researchers is to use an artificial arm to demonstrate the effects of severe collisions. With this kind of information and research, they hope to "cap" the maximum speed a robot should travel when it senses a human nearby in order to prevent injury.